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J. Phys. Chem. C 2008, 112, 13692–13699
Visible Light Photocatalytic Activities of TiO2 Nanocrystals Doped with Upconversion Luminescence Agent Guangjian Feng,† Suwen Liu,*,† Zhiliang Xiu,† Yin Zhang,‡ Jiaoxian Yu,† Yonggang Chen,† Ping Wang,† and Xiaojun Yu† Department of Materials Science and Engineering, Shandong Institute of Light Industry, Jinan 250353, P. R. China, and Department of Physics, Shandong Institute of Education, Jinan, Shandong 250013, P. R. China ReceiVed: March 20, 2008; ReVised Manuscript ReceiVed: June 23, 2008
In order to enlarge the utilization ratio of the visible light, nanometer TiO2 photocatalysts doped with Er3+: Y3Al5O12 (Er3+:YAG) upconversion luminescence agent were synthesized. The as-synthesized samples were characterized by X-ray diffraction, transmission electron microscopy, and ultraviolet-visible absorption spectra. The visible light photocatalytic activities of the samples were studied by photodegradation of methylene blue while considering the influences of the dopant concentration, calcination temperature, initial concentration of methylene blue, and catalyst amount. The results showed that the degradation rate in the presence of the doped sample was 2 times that of the undoped one. The photocatalytic mechanics of the degradation reaction was investigated. The results showed that primarily the oxidation mechanics of the photoproduction holes occurred, and the methylene blue was degraded thoroughly in the presence of the doped samples. All the results proved that the doped TiO2 had more excellent photocatalytic activities than the undoped one, and this method can be envisaged as a technology for treating dye wastewater using solar irradiation. 1. Introduction TiO2 is known as one of the most effective photocatalysts for the degradation of inorganic and organic pollutants, and its photocatalytic behavior has been studied extensively.1-4 TiO2 is capable of decomposing a wide variety of inorganic and organic pollutants and toxic material in both liquid and gas phase systems, yet the energy gap of pure anatase TiO2 is about 3.2 eV, so only ultraviolet light (λ < 387 nm, about 4% of the solar light) can be absorbed by pure anatase TiO2.5,6 It is of interest to find a TiO2-based photocatalyst which is sensitive to visible light in order to make more efficient use of solar energy in practical applications. There are several methods to develop that offer promise for photocatalysts for wastewater purification, such as surface modification,7-9 metal 10-13 or nonmetal ion 14-21 doping, generation of an oxygen vacancy,22,23 expensive ion-implantation facility and complex treated processes,24 and the combination with other semiconductors. 25-28 Recently much attention has been paid to extend TiO2 photoresponse range by using modified methods. According to the formula Eg ) pν (p is Planck constant, ν is light frequency), the photopotential energy excited by visible light is lower than that excited by ultraviolet light. The photoinduced energy excited by visible light only decomposes azo bonds of some dye compounds (such as methyl yellow, methyl red, and crystal violet) or makes dye compounds decolorized, while the photoinduced energy excited by ultraviolet light can degrade dye compounds completely.29,30 Thus, the upconversion luminescence agent that could transform the visible light into the ultraviolet light to satisfy the genuine requirement of TiO2 catalysts became the investigated subject.31 Wang et al. have reported that a TiO2 catalyst doped with an amorphous upcon* Corresponding author. Tel.: +86-531-89631231. Fax: +86-53189631227. E-mail:
[email protected]. † Shandong Institute of Light Industry. ‡ Shandong Institute of Education.
version luminescence agent has a much higher photocatalytic activity than the undoped one.32 However, the luminous power of the amorphous upconversion luminescence agent is less than that of the crystalline one.33 So far, almost all studies have been focused on the adsorption 34-36 and degradation kinetics 37-39 of the photocatalytic reaction. There are only a few research studies about the effects of different oxidation mechanisms on degradation. In this paper the crystalline upconversion luminescence agent (Y3Al5O12 doped with Er3+, Er3+:YAG) and the TiO2 photocatalysts doped with Er3+:YAG or YAG were prepared. The photocatayst property of the samples for decomposing the methylene blue was examined in detail, and the oxidation mechanism was evaluated according to the change of the absorption peaks in different wavelength regions. 2. Experimental Section 2.1. Experimental Materials. Methylene blue (98% purity), Er2O3 (99.999% purity), Al(NO3)3 (analytical purity), citric acid (analytical purity), Y2O3 (99% purity), and P25 (Degussa Germany) were used. 2.2. Preparation of the Samples. First, 5 g of mixed powders of Y2O3 and Er2O3 was milled in ethanol adequately for 1 h and then dissolved into an appropriate amount of boiling HNO3 with the molar ratio of Al(NO3)3, Y2O3, and Er2O3 fixed at 100:30:0.25. Second, solution A was prepared by dissolving proper quantities of citric acid into the nitrate solution of Y3+ and Er3+; solution B was prepared by dissolving Al(NO3)3 into an appropriate amount of distilled water. Third, a gel was obtained by adding solution B into solution A under magnetic stirring at 40 °C. A xerogel was obtained by drying the gel at 100 °C for 24 h. Finally, the Er3+:YAG was obtained by calcination of the prepared xerogel at 900 °C for 3 h. Mixed powders were prepared by milling the mixture of Er3+: YAG or YAG powder and P25, and then TiO2 catalysts doped with Er3+:YAG or YAG were prepared by calcining the mixed
10.1021/jp802476t CCC: $40.75 2008 American Chemical Society Published on Web 08/12/2008
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Figure 2. XRD of the samples: (A) Er3+ doped YAG; (B) TiO2 containing 24 mol % YAG; (C) TiO2 containing 16 mol % YAG; (D) undoped anatase TiO2.
Figure 1. (a) Excitation spectra of upconversion luminescence agent (a, 450 nm; b, 465 nm; c, 476 nm; d, 500 nm). (b) Emission spectra of upconversion luminescence agent under excitation of 450 nm wavelength light (a, 2P3/2 f 4I15/2 (326 nm); b, 2I11/2 f 4I9/2 (348 nm); c, 4 G11/2 f 4I15/2 (363 nm)).
powders at 500 °C for 3 h. The molar ratio of the Er3+:YAG or YAG powder to P25 was 0, 1, 2, 3, 4, 8, 12, 16, 20, and 24%, respectively. 2.3. Experimental Procedures and Techniques. A 220 V/40 W commercial fluorescence lamp (440 nm < λ < 650 nm) was used as the light source. The experiments were carried out in an 80 mL beaker with an amount of photocatalyst in proper methylene blue solution. The distance between the liquid surface and the light source was about 6.5 cm. The degradation rate of the as-synthesized samples was estimated according to formula η ) (A0 - At)/A0 × 100% (η: degradation rate of methylene blue; A0: 510 nm absorption peak before irradiation; At: 510 nm absorption peak after irradiation). The upconversion luminescence property was detected by a spectrofluorimeter (F-4500, 150 W Xe lamp, Hitachi Company, Japan). The X-ray diffraction (XRD) pattern of the powder was examined using the Japan RigaKu D/max-γ X-ray diffractometer system with graphite monochromatized Cu KR irradiation (λ )1.5418 Å). The transmission electron microscopy (TEM) readings were taken using a JEM-100CX transmission electron microscope running at an accelerating voltage of 100 kV. The ultraviolet-visible light absorption spectra (UV-vis) were measured using a Shimadzu UV-757CRT spectrophotometer. 3. Results and Discussion The fluorescent excitation spectrum of the Er3+:YAG sample is shown in Figure 1a, from which four fluorescent excitation peaks (a, 450 nm; b, 465 nm; c, 476 nm; d, 500 nm) can be found. The results indicate that the visible light in the range of 400-550 nm can be absorbed by Er3+:YAG. The fluorescent emission spectrum excited by visible light of 450 nm shows
three emission peaks in the region of 320-380 nm (Figure 1b), which correspond with (a) 326 nm (2P3/2 f 4I15/2), (b) 348 nm (2I11/2 f 4I9/2), and (c) 363 nm (4G11/2 f 4I15/2), respectively. The wavelengths of the three emission peaks are shorter than 380 nm, which proves Er3+:YAG can supply the ultraviolet light the TiO2 needs. XRD patterns of Er3+:YAG are shown in Figure 2A. The Er3+:YAG basically retains cubic crystal form; we cannot find the Er2O3 diffraction peaks in the figure. The doped (Figure 2B,C) and undoped TiO2 samples (Figure 2D) basically retain anatase crystal form, but two diffraction peaks belonging to the YAG cubic crystal form can be found in the pattern of the doped ones. The effect of the dopant concentration on peak intensity is also shown in Figure 2. From the diffraction peak intensity and width it can be concluded that the TiO2 grain sizes of the doped samples are less than that of the undoped ones, which may be due to some impurity ions doped into the TiO2 crystal surface in the calcination process to restrain the growth rate of TiO2 crystal grain.40 TEM images of the samples are shown in Figure 3. It can be found that the undoped TiO2 particles with a well dispersibility are all about 30 nm (Figure 3A), and the Er3+:YAG particles assume reticulation on the whole spread at 30-80 nm (Figure 3B). UV-vis spectra of the doped samples are shown in Figure 4, which indicate that under the low dopant concentration, the samples doped with Er3+:YAG or YAG had the same absorption band at the place of K1 (about 387 nm), this absorption band can be ascribed to the property of TiO2. When the concentration increased gradually, no obvious variation could be found in Figure 4b, but another absorption band K2 (480 nm) could be found in Figure 4a. The Figure 4c indicates that the Er3+:YAG has the ability to absorb visible light, but YAG has not. So when the dopant concentration became high, the samples doped with Er3+:YAG appeared another absorption band at visible light region, which also indicates that the samples doped with Er3+: YAG has the photoresponse property in the visible light region. The influence of the dopant concentration on degradation rate is shown in Figure 5a. The degradation rate reaches its maximum (11.4%) at the dopant concentration of 16 mol % (the molar ratio of Er3+:YAG to P25 is 16%) under visible-light irradiation for 2 h, and then it decreases with the increase of dopant concentration. The amount of ultraviolet light transformed by Er3+:YAG increases with the dopant concentration, so the photocatalytic activity of doped samples increased with dopant concentration when the concentration was low. But the amount
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Figure 3. TEM of the samples: (A) undoped TiO2; (B) upconversion luminescence agent.
of TiO2 decreases with the increase of the dopant concentration, so the activity decreased when dopant concentration exceeded a certain value. In this experiment, the optimal dopant concentration is 16 mol %. But the degradation rate of the sample doped with YAG has no obvious change with the dopant concentration, which proves the Er3+ has an effective activity in the YAG. Figure 5b shows the influence of the calcination temperature on degradation rate. The photocatalytic activity increases first and then decreases with the calcination temperature under visible-light irradiation for 2 h, which can be attributed to Er3+: YAG and TiO2 having not bonded better to make good use of the UV light at the low calcination temperature. When the calcination temperature was high, the TiO2 grain size became large to go against photocatalytic activity, so the doped samples had an optimal calcination temperature. In this experiment, the optimal calcination temperature is 500 °C. The influence of the initial concentration of methylene blue solution on degradation quantity was researched (Figure 5c). The initial concentration has a significant effect on the degradation quantity. The degradation quantity reaches its maximum (0.24 mg) at 30 mg/L under visible-light irradiation for 5 h, and then it reduces with the increase of the initial concentration. It can be ascribed to the fact that the amount of methylene blue increased with the initial concentration when the initial concentration was low; thus the adsorption rate of the TiO2 in methylene blue increased and the photocatalytic activity increased. But when the initial concentration surpassed 30 mg/L, a further increase in concentration caused light scattering and absorbing, so the light quantity irradiated on the catalytic surface was reduced.41 In addition, so much methylene blue adsorbed on the TiO2 surface also had an adverse effect on photocatalytic activity.42 In this experiment, the optimal initial concentration is 30 mg/L. The influence of the photocatalyst amount on degradation was investigated (Figure 5d) by varying the amount of methylene
Feng et al. blue solution from 0.03 to 0.18 g/30 mL. The degradation rate reaches its maximum at 0.12 g/30 mL under visible-light irradiation for 4 h, and then it decreases as the photocatalyst amount increases. When the amount of methylene blue solution was little, the degradation rate increased with the amount because of an increased number of available adsorption and catalytic sites on the catalyst surface. When the photocatalyst amount surpassed 0.12 g/30 mL, degradation rate decreased because a further increase in catalyst loading caused light scattering and a screening effect and reduced the specific activity of catalyst. Part of the catalyst surface probably became unavailable for photon absorption and dye adsorption under such conditions, thus bringing little stimulation to catalytic reaction.43 The results suggest that an optimal amount of the catalyst is necessary for enhancing the degradation rate and reducing needless waste, and in this experiment the optimal amount is 0.12 g/30 mL. Finally, a detailed study of the reaction kinetics on photocatalytic degradation was investigated. Three absorption peaks can be found in the UV-vis spectra of the methylene blue (M) solution, which correspond to (A) 570 nm (MA), (B) 300 nm (MB), and (C) 250 nm (MC), respectively. According to the molecular formula of methylene blue (Figure 6), it can be concluded that MA corresponds with the chromophore position, and MB and MC correspond with the positions containing the benzene ring. Figure 7 indicates that there is a similar peak intensity variety for the peaks in the UV light region, so MB and MC are regarded as one peak (MB) in a later discussion for convenience. In the degradation process, TiO2 excited the photoproduction hole (h+) and the photoproduction electron (e-). The h+ excited by the UV light has a strong oxidizing property (3.2 eV) which could react with H2O to generate hydroxyl free radical · OH (2.8 eV). All of the h+, e-, and · OH enter into the degradation reaction: h+ could oxidize organic matters by capturing the electrons of the covalent bond and cause the covalent bond to be destroyed directly; e- can react with the chromophore position; · OH is a kind of chemical reaction active material, which could react with organic matter through dehydrogenation, replacement, and addition reactions.44 Thus many kinds of intermediates would be generated in the degradation. The reaction of the degradation can be described as follows:45
TiO2 + pν f e- + h+ h+ + H2O f H+ + · OH D+Cl- represents methylene blue, and the reaction of e- and methylene blue can be described as follows:
e- + D+Cl- f D + Cle- + O2 f O2- + HO2 f O2 + H2O2 + e- f OH- + · OH · OH + D+Cl- f D-OH + ClC6H6-X represents the organic-containing benzene ring, and the reaction of · OH and C6H6-X can be described as follows:46
· OH + C6H6-X f HO-(C6H6)-X (MBR) The reaction of h+ and C6H6-X can be described as follows:46
h+ + C6H6-X f (C6H6)+-X (MX) This process indicates that the intermediates of different reaction mechanics are different. h+ can destruct the benzene
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Figure 4. UV-vis spectra of the doped samples. (a) Samples doped with Er3+:YAG; doped concentration: (A) 24%, (B) 20%, (C) 16%, (D) 12%, (E) 8%, (F) 4%, (G) 3%, (H) 2%, (I) 1%, (J) 0%. (b) Samples doped with YAG; doped concentration: (A) 4%, (B) 8%, (C) 0%, (D) 16%. (c) (A) Er3+:YAG, (B) YAG.
Figure 5. Influence of different elements on the degradation rate of methylene blue solution: (a) influence of different dopant concentration samples on the degradation rate (2 h); (b) influence of different calcination temperatures on the degradation rate (2 h); (c) influence of the initial concentration of methylene blue solution on the degradation rate (5 h); (d) influence of an additional amount of photocatalyst on the degradation rate (4 h).
ring structure directly, e- can only destroy the chromophore position, and · OH only has an addition reaction to the benzene ring. e- and · OH could not destruct the benzene ring and could only affect the chromophore position (MA), so e- and · OH were both seen as · OH reaction mechanics in later discussion.
Figure 7a1 shows that MA decreases with irradiation time under ultraviolet-light irradiation in the presence of TiO2, which indicates that the chromophore position was destroyed in the degradation. But MB increases quickly in a short time after the reaction started and then decreases gradually with irradiation
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Figure 6. Structure of methylene blue.
time. This is due to · OH reacting with C6H6 to generate HO-(C6H6).
· OH + C6H6 f HO-(C6H6)
(I)
MBR are generated, but the doped sample still holds high activity, and almost no MBR is generated. This paper considers that only a small part of visible light was absorbed by some position on the TiO2 surface which contains many flaws and vacancies under visible-light irradiation.48 The h+ excited by this part of light was too little to degrade a major part of the organic around TiO2 and destroy the adsorption-desorption balance in a short time, so the chief reaction follows mixed oxidation mechanics of · OH and h+. However, the h+ excited by visible light has less oxidizability (
